A number of agents are currently known which exhibit a beneficial effect upon feed consumption and utilization in animals when administered in conjunction with normal feed regiments. Some of the most important agents are those which increase the output by domestic animals of products utilized by humans. For example, ruminant animals, such as cattle, sheep and goats, experience an increase in the efficiency of their feed utilization as well as body growth promotion, and wool growth promotion in sheep, when administered effective amounts of growth promotors such as monensin, salinomycin, lasalocid and related compounds. Many of the growth promotors operate by altering the breakdown of food in the rumen of the animal. To be effective, such agents must be deposited in the rumen or reticulo-rumen of the animal.
A major problem associated with drug administration to animals in general is the frequency required and the resulting rise in labor costs necessitated thereby. Moreover, many animals utilized for human food consumption, particularly ruminants such as cattle and sheep, are range fed for extended periods of time prior to feed lot development, thus rendering drug administration by daily dosing or by feed additives virtually impossible, and economically impracticable.
The administration of drugs to animals and humans by prolonged release formulations is known to be effective in some instances. For example, it is known to deposit a drug in an impregnable strip to be worn as a collar around the neck of an animal. The drug slowly released for the prolonged treatment of external parasites. Reuter et al., in U.S. Pat. No. 4,011,312, discloses a prolonged release drug dosage form useful for the treatment of bovine mastitis. Such formulation is comprised of a suitable antimicrobial agent dispersed in a copolymer made up of about 60 to 80 mole percent glycolic acid and 20 to 40 mole percent lactic acid. Such copolymer is said to have a molecular weight of less than 2000. While the copolymer is effective in slowly releasing a therapeutic agent when in contact with the fluids of the teat canal, such copolymer is ineffective for beneficial controlled release when subjected to the fluids of the rumen.
An object of the present invention is to provide formulations designed for controllably releasing an effective amount of growth promotant to a ruminant over a prolonged period of time. A further object of the invention is to provide a method for increasing feed utilization in ruminants. Such method is particularly important in the treatment of range fed animals, since a single treatment according to this invention is effective for several months.